This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The abbreviations and terms appearing in the description and drawings are defined as below.    3 GPP Third Generation Partnership Project    AIR Antenna Integrated Radio    BS Base Station    DL Downlink    FDD Frequency Division Duplex    LTE Long Term Evolution    LTE-A Long Term Evolution-Advanced    RMS Root Means Square    RRU Remote Radio Unit    SPDT Single Pole Double Throw    TDD Time Division Duplex    TDSCDMA Time Division-Synchronous Code Division Multiple Access    TOR Transmitter Observation Receiver    UL Uplink    VSWR Voltage Standing Wave Ratio
Smart antenna has been one of the most important features in the telecommunication Time Division Duplex (TDD) standards and also starts in Frequency Division Duplex (FDD) standards. A base station can use a smart antenna array to increase its maximum range and capacity due to an improved antenna gain and the reduction of interference from other spatially separated users. Beamforming algorithms often assume that the antenna array has no errors and that the multi-channel transceiver has an identical transfer function. This feature requires “antenna calibration” to make sure that the gain/amplitude and phase of each channel/path can be known and controlled.
FIG. 1 shows a brief block diagram of a normally used solution for antenna calibration. As shown in FIG. 1, there is a radio unit 110, an antenna array 120, and multiple pairs of radio frequency (RF) ports 112 between the radio unit 110 and the antenna array 120. The pairs of RF ports 112 are connected by RF cables 130. The radio unit 110 delivers RF signals via the RF ports 112 to the antenna array 120 for transmitting, and receives via the RF ports 112 RF signals received by the antenna array 120 from the air. The radio unit 110 may comprise multiple transmit/receive (TX/RX) paths/channels (not shown) corresponding to the RF ports 112. Each TX/RX path may include various components for performing signal processing in the RF domain, for example various amplifiers such as low noise amplifier (LNA) or high power amplifier (HPA), digital-to-analog converter (DAC), analog-to-digital converter (ADC), modulator, mixer, and etc.
For calibrating antennas, a pair of RF ports is provided specially for antenna calibration, i.e., antenna calibration ports 114. An antenna coupling unit 122 is arranged within the antenna array 120 to couple calibration signals into/out of the multiple TX/RX paths.
FIG. 2 illustrates the signal flow for uplink (UL) calibration based on the hardware structure in FIG. 1. The UL calibration may also be referred as receiver (RX) calibration.
As shown in FIG. 2, a known UL calibration signal 150 is transmitted through the antenna calibration ports 114 to the antenna coupling unit 122. The UL calibration signal may be generated by some device separated from the radio unit 110, for example, a UL calibration signal generator arranged within a baseband unit (not shown). The antenna coupling unit 122 can inject the UL calibration signal into one RX path to be calibrated. Then, the UL calibration signal travels through the RX path and arrives at a RX calibration unit (not shown). The RX calibration unit can process the UL calibration signal and estimate the transfer function of the RX path. The algorithm for estimating the transfer function can be any known algorithm in the art
It should be noted that, only one RX path calibration is shown here and other paths are identical. The multiple RX/UL paths may be calibrated simultaneously or sequentially. The antenna calibration ports 114 are served as the common transmitter for all RX paths.
FIG. 3 illustrates the signal flow for downlink (DL) calibration based on the hardware structure in FIG. 1. The DL calibration may also be referred as transmitter (TX) calibration.
As shown in FIG. 3, a known DL calibration signal 140 is travelling through one TX path. Similarly, the DL calibration signal may be generated by some device separated from the radio unit 110, for example, a DL calibration signal generator arranged within a baseband unit (not shown). The antenna coupling unit 122 can extract the DL calibration signal from the TX path to be calibrated, and feed it back via the antenna calibration ports 114 to a TX calibration unit (not shown). The TX calibration unit can process the DL calibration signal and estimate the transfer function of the TX path.
It should also be noted that, only one TX path calibration is shown here and other paths are identical. The multiple TX/DL paths may be calibrated simultaneously or sequentially. The antenna calibration ports 114 are served as the common receiver for all TX paths. In other words, the antenna calibration ports 114 act as reference uplink (UL) channel for all DL signals.
In addition, antenna supervision is a very traditionally required feature to detect if the antenna is well connected. Voltage Standing Wave Ratio (VSWR) is normally used for antenna supervision. When a transmission line (cable) is terminated by an impedance that does not match the characteristic impedance of the transmission line, not all of the power is absorbed by the termination. Part of the power is reflected back along the transmission line. The forward (or incident) signal mixes with the reverse (or reflected) signal to cause a voltage standing wave pattern on the transmission line. The ratio of the maximum to minimum voltage is known as VSWR. Thus, in antenna supervision, the forward (or incident) power and the reverse (or reflected) power at the antenna ports will be detected to supervise the connection status between the radio unit and the antenna array.
FIG. 4 briefly shows a normally used antenna supervision solution. As shown in FIG. 4, a coupler 116 is coupled near the RF port 112 at the radio unit side. The coupler 116 extracts the forward signal and the reverse signal from the transmission line in question to a RMS power detection unit 118. Normally, the RMS power detection unit 118 may be implemented by a simple receiver, which may includes a down-converter, a frequency generation unit and an ADC to make sure that the power can be accurate with the presence of interferers. It should be noted that, only one path configured with a coupler 116 is shown here and other paths are identical.